1. Field of the Invention
This invention relates to synchronization between systems. More specifically, this invention relates to the synchronization of electronic article surveillance systems through the use of an RF synchronization signal.
2. Description of the Relevant Art
Electronic Article Surveillance (EAS) systems are detection systems that allow the identification of a marker or tag within a given detection region. EAS systems have many uses, but most often they are used as security systems for preventing shoplifting in stores or removal of property in office buildings. EAS systems come in many different forms and make use of a number of different technologies.
A typical EAS system includes an electronic detection unit, markers and/or tags, and a detacher or deactivator. The detection unit is used to detect any active markers or tags brought within the range of the detection unit. The detection units can, for example, be bolted to floors as pedestals, buried under floors, mounted on walls, or hung from ceilings. The detection units are usually placed in high traffic areas, such as entrances and exits of stores or office buildings.
The markers and/or tags have special characteristics and are specifically designed to be affixed to or embedded in merchandise or other objects sought to be protected. When an active marker passes through the detection unit, the alarm is sounded, a light is activated, and/or some other suitable control devices are set into operation indicating the removal of the marker from the proscribed detection region covered by the detection unit.
Most EAS systems operate using the same general principles. The detection unit includes a transmitter, which is placed on one side of a detection region and a receiver, which is placed on the opposite side of this detection region. The transmitter sends a signal at defined frequencies across the detection region. For example, in a retail store the detection region is usually formed by placing the transmitter and receiver on opposite sides of a checkout aisle or an exit. When a marker enters the region, it creates a disturbance to the signal being sent by the transmitter. For example, the marker may alter the signal sent by the transmitter by using a simple semiconductor junction, a tuned circuit composed of an inductor and capacitor, soft magnetic strips or wires, or vibrating resonators. The marker may also alter the signal by repeating the signal for a period after the signal transmission is terminated by the transmitter. This disturbance caused by the marker is subsequently detected by the receiver through the receipt of a signal having an expected frequency, the receipt of a signal at an expected time, or both. As an alternative to the basic design described above, the receiver and transmitter units, including their respective antennas, can be mounted in a single housing.
One key concern with EAS systems from a design standpoint is ensuring that there is proper synchronization as between the transmitter and the receiver. For example, in many systems it is highly important that the transmitter window, during which time the transmitter transmits a marker exciter signal, does not overlap with the receiver window, during which the receiver is attempting to detect a marker response signal. In these systems, any overlap between these two windows will result in degradation of system performance. Typically, these two windows are separated by an off state during which neither the receiver or the transmitter is active.
Certain conventional EAS systems rely on a local power line current or voltage zero crossing for synchronization of the transmitter window and the receiver window. If there is no other EAS system in close proximity, then the actual position of the transmit and receive windows versus the power line zero crossing is not very important. However, when more than one such system is installed at a distance which allows the receiver of one system to receive a transmitter signal of another system, then the relative position of the transmit and receive windows in all systems becomes very important. Such a situation may occur for example when there are multiple exits which require separate EAS systems. If the power line zero crossings for all of the EAS systems happen at the same time then the transmit and receive windows of all of the EAS systems will be synchronized relative to one another. In that case, all windows are perfectly aligned, and there is no possibility that the transmitter pulse of one system will be seen in the receiver of another system. More often however, the various EAS systems are connected to different power line outlets, each having a unique power line phase shift related to the type of load on the power line. This phase shift can vary over time and can cause the transmit and receive windows of the various EAS systems to overlap, resulting in degraded performance or false alarming.
Prior art systems have made use of an off state to delay the time between the transmitter and receiver windows. This approach allows for a small phase shift between nearby EAS systems while still ensuring that there is no overlap between transmit and receive windows of the nearby systems. However, this is not an entirely satisfactory solution to the problem. This is partly due to the fact that time must be allowed for the transmitter to transition from an on state to an off state. In any case, significantly extending the off state to accommodate larger phase shifts between power line zero crossings is not practical because the signal from a tag or marker starts to decay as soon as the transmitter pulse is removed. Delaying the receiver window relative to the transmitter pulse reduces the received marker signal and therefore limits the range of detection for the system.